Fix mutability of get_var

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "embed-nu"
-version = "0.3.4"
+version = "0.3.5"
 edition = "2021"
 license = "Apache-2.0"
 repository = "https://github.com/Trivernis/embed-nu"

src/context/mod.rs

@@ -55,7 +55,7 @@ impl Context {
     }
 
     /// Returns a variable defined in the stack
-    pub fn get_var<S: AsRef<str>>(&mut self, name: S) -> Option<nu_protocol::Value> {
+    pub fn get_var<S: AsRef<str>>(&self, name: S) -> Option<nu_protocol::Value> {
         let name = name.as_ref();
         let dollar_name = format!("${name}");
         let var_id = self